Abstract: A tuning aid for tuning musical instruments, particularly keyboard instruments, includes a sensor unit (12) which has an elongate capacitative sensor probe which is capable of extending across one octave of the instrument, and an octave switch (22) and a tone switch (20) on which are manually set the nominal octave and note of the string being measured. Amplifiers (16,24) including an AGC amplifier and controlled band-pass filters (18,26) are responsive to the octave and tone switches to select the measured tone. A divider (32) responsive to the octave switch divides the measured frequency by a power of two. A divider (38) is connected to the output of a crystal oscillator (36) to divide by a factor dependent upon the tone switch setting. The outputs of the two dividers are compared in a discriminator (34) which compares the two frequencies. A meter (50) displays the sense and magnitude of the discriminator output so as to display in terms of cents the amount by which the frequencies differ.